Apparatuses for stabilizing an on-board motor vehicle electrical system are known in principle. The problem faced during a starting process in a motor vehicle is, in principle, that the starter of the motor vehicle requires a high current which it draws from the vehicle battery. The high level of current drawn by the starter leads to voltage dips at the battery terminals, this leading to a voltage dip in the entire on-board electrical system. All electronic components and devices of the motor vehicle are consequently affected by a voltage dip of this kind. The depth of a voltage dip, which all components and devices which are connected to the on-board electrical system have to withstand without interference in their function and without being damaged, is defined by the manufacturers of the respective components. However, voltage dips which are even deeper than those specified by the manufacturer may occur at the battery terminals for various reasons, for example owing to a weakly charged, a damaged or an old battery. Deep voltage dips of this kind can lead to functional interference, as far as damage to the components and devices which are connected to the on-board electrical system. It is therefore necessary in every case to provide for voltage limiting at the battery terminals or to limit the starter current such that the voltage at the battery terminals cannot drop below a prespecified threshold value.
Known methods for limiting voltage can be subdivided substantially into passive methods and active methods. Passive methods for limiting voltage use resistors, which are inserted between the battery and the starter, in order to limit the starter current. Passive methods of this kind can be implemented in a particularly simple manner, but cause high-energy losses and are relatively inflexible because they have to be matched to a specific battery type and consequently have to be redefined and replaced when the battery is changed. Active methods for limiting voltage usually use pulse-width modulation (PWM) in combination with resistors, semiconductors and/or other components.
Irrespective of the type of voltage dip-limiting method, a voltage dip-limiting measure is only introduced in the case of the known apparatuses when a voltage dip has been detected in the on-board electrical system. The apparatus which detects the voltage dip then sends a corresponding control signal to an apparatus for stabilizing the on-board motor vehicle electrical system, said apparatus consequently initiating a suitable measure for limiting the voltage dip. This measure involves, for example, limiting the starter current and/or connecting an additional energy source.
One disadvantage of the known apparatuses is therefore that a voltage dip in the on-board electrical system is actively countered only after said voltage dip is detected, that is to say only in the event of a fault. It is therefore necessary to first generate a control signal on the basis of a voltage dip in order to activate monitoring of the battery voltage and subsequently further to limit the voltage dip which has already taken place. Therefore, under certain circumstances, there may be a period of time between a voltage dip in the on-board electrical system and the activation of the voltage dip-limiting measure which may possibly already be long enough for components and devices to be damaged on account of the voltage dip.